1. Field of the Invention
The invention relates in general to AC power regulation. More particularly, the invention pertains to constant current regulators using active solid state switch devices.
2. State of the Art
Solid state insulated gate bipolar transistor (IGBT) switches capable of handling large current loads have recently become available, enabling a number of applications for pulse width modulation control which previously were impractical. Previous switching technologies for high currents were generally limited to three types of switches, one of which is the silicon control rectifier (SCR) switch, which can be turned on with a control signal but must turn off with current reversal. A power metal oxide silicon field effect transistor (MOSFET) has also been used, but tends to waste significant quantities of power because of its resistive characteristics. Bipolar transistors have also been used, but require large base currents to supply large output currents. On the other hand, the IGBT type of switch requires much less drive current and has a nearly constant saturation voltage as current increases, requiring less power consumption than the MOSFET switch, for example.
Existing constant current regulators (CCR) used for airport lighting systems have usually consisted of one of three types: saturable core regulators, resonant network regulators, and pulse width modulation (PWM) regulators. The saturable core regulators and resonant network regulators have significant efficiency and size disadvantages in comparison to PWM regulators. Two types of PWM regulators are known in the prior art, those based on silicon controlled rectifiers (SCR) and those based on IGBT.
The SCR-based regulators use silicon controlled rectifiers which are triggered partway through a 60 Hz input current cycle and remain ON until the current reverses. The width of the ON time determines the amount of current in the primary winding of the transformer and can be varied to control the output current. This approach is reasonably efficient, but has a significant drawback in that the system power factor is generally much less than the ideal 1.0, because the current always lags the voltage and the shape of the current waveform causes high levels of harmonics in the input current.
Regulators based on IGBT have overcome limitations of the SCR-based regulators by switching ON and OFF many times per cycle, usually at about 50 to about 100 times per cycle. This permits the power factor to be maintained at a high level while providing excellent controllability of the output current. The biggest drawback to IGBT-based regulators is that the arrangement of the IGBT transistors requires carefully controlled timing between the IGBT control signals. If the timing of input IGBT and catch IGBT is too close, excessive current will be dissipated by the IGBT's and the input can experience excessive momentary short circuits. On the other hand, if the timing between the input IGBT and catch IGBT is overly separated, excessive voltages can build up on the IGBT transistors, or the efficiency of current transfer from the primary winding to the secondary winding will be reduced. Furthermore, the required timing may vary as a function of temperature, current, or duty cycle, making control of this timing very difficult and possibly leading to failure of the controller.
It is therefore a primary object of the present invention to provide a constant current regulator which provides a controllable uniform current while eliminating the difficult timing constraints common to prior IGBT regulators. Another object of the invention is to provide a constant current regulator which is controllable over a wide range of load current without failure. It is a further object of the invention to provide a constant current regulator in which power dissipation is minimized. A still further object of the invention is to provide a reliable constant current regulator formed of commercially available components at low cost.